Extreme changes in cellular architecture are experienced by germ cells during spermatogenesis. The most extensive of these changes take place after meiosis, when the chromatin is condensed, the nuclei are dramatically reshaped, the mitochondria fuse into a single giant organelle, and extensive reorganization of the cytoplasm must be accomplished. Correspondingly, the majority of male-sterile mutations in Drosophila appear to affect postmeiotic steps in spermatogenesis. The reorganization of spermatid nuclei in particular may be a critical determinant of male fertility, as several studies argue for a clear correlation between sperm nuclear morphology and fertility. While a large number of Drosophila genes are mutable to "late" male-sterile phenotypes, in which highly elongated bundles of sperm fail to be properly matured, most of these mutations appear to lie within genes whose functions are required in other tissues and stages of development. Genes whose functions are required only in spermiogenesis may be of considerable interest for the understanding of the genetic logic by which spermiogenesis proceeds, and for the development of chemotherapeutic strategies that will specifically target this process with a minimum of effects on other tissues and physiological pathways. Preliminary evidence suggests that a previously undescribed Drosophila gene, sad sack, is a hotspot for mutations affecting spermatid maturation, is required only for male fertility, and may play a role in the nuclear reorganization of spermatids. The detailed spermatogenic defect of sad sack mutants will be determined through a combination of fluorescence and electron microscopy, with particular attention to the structure, composition, and dynamics of the nuclear envelope. The sad sack gene, currently localized to a chromosomal region of 320 kb, will be mapped to a fine-structure level by male recombination and/or strategies based on the generation of small overlapping deletions throughout the region. Candidate gene(s) identified through this approach will be analyzed in more detail by examining their expression throughout development, screening for molecular lesions in candidate genes, and tested for genetic rescue of sac mutants by germline transformation. For localization of the sac gene product within developing sperm, antibodies will be generated against synthetic peptides of amino acid sequence unique to sac or bacterially-expressed sac protein.